I am building a room in my backyard on a slab a mere 5 1/2" from the block walls on 2 sides. I'm planning on building those 2 walls (15' by 6' and 8' x 6' sloping to 7') on the floor, with Tyvek (or ?) then prefinished siding, then lifting them onto the bolts.
My dilemma is how do I seal that corner?

Any recommendations on the unseen siding (vinyl, cement, aluminum, ).
Any unforeseen problems here?

Thanks in advance.

Are the corners of the new walls (at the exterior side) going to be flush with the corners of the existing structure? If so... maybe you can add a piece of trim that will overlap the existing wall by a few inches and before you set the new wall in place... apply a generous amount of silicone sealant to the trim so that when you pull the new wall snug with the existing wall, it creates a seal. Also... I would suggest injecting some expanding foam insulation from the inside of the new wall along the edge that abuts the existing wall.